In the oil and gas industry, seismic prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. In seismic prospecting, a seismic source is used to generate a seismic signal which propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different elastic properties). The reflected signals (known as "seismic reflections") are detected and recorded by seismic receivers located at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the subsurface formations.
Seismic prospecting consists of three separate stages: data acquisition, data processing, and data interpretation. The success of a seismic prospecting operation depends on satisfactory completion of all three stages.
The seismic energy recorded by each seismic receiver during the data acquisition stage is known as a "seismic data trace." Seismic data traces often contain both the desired seismic reflections (the "primary" reflections) and unwanted multiple reflections which can obscure or overwhelm the primary seismic reflections. A primary reflection is a sound wave that passes from the source to a receiver with but a single reflection from some subsurface seismic reflector. A multiple reflection is a wave that has reflected at least three times (up, down, and back up again) before being detected by a receiver. A surface-related multiple reflection is a multiple in which at least one downward reflection occurs at the surface of the earth (onshore survey) or at the air-water interface (offshore survey). An interbed multiple has each downward reflection occurring from a subsurface reflector. Clearly, multiples contain no useful information that is not more easily extracted from primaries. Worse, the signals from multiples obscure that from the primaries, making the primaries hard to identify and interpret. For that reason, removal of multiples, or at least attenuation of multiples, by one method or another, is a necessary part of the seismic data processing stage in many environments, particularly in marine settings where multiples are especially strong relative to primaries. This is due mostly to the fact that the air-water reflection coefficient is almost unity.
Velocity analysis is the process of calculating stacking velocities from measurements of the normal moveout of seismic data. Traditional processing for velocity analysis requires that some person (the processor) decide, based on velocity stack plots and his experience in the area of the survey, what is primary and what is multiple in a seismic section. This process is subject to personal interpretation and can be very time consuming.
Current multiple attenuation techniques can be roughly divided into two categories; filtering methods and wave-equation prediction methods. Filtering methods rely on periodicity of the multiples or on significant velocity differences between primaries and multiples. Predictive deconvolution is a filtering method that assumes that multiples are periodic while primaries are not. This assumption is usually met for data from water depths less than 500 msec (approximately 1,200 feet) and approximately layered subsurface geology. In areas of water depths greater than 500 msec where the velocity difference between primaries and multiples are significant, velocity-filtering methods such .tau.-p and f-k filtering can be used. The variable f represents frequency, k is the wavenumber, p is the ray parameter, and .tau. is the zero offset intercept time.
Wave-equation methods use the physical wave-propagation phenomenon to predict and subtract multiples from data. Wave-equation methods can be very accurate, but also very expensive and time consuming to use compared to filtering methods. Wave-equation methods exploit the fact that primaries and multiples are physically related. These methods can handle complex geometries and need little or no information about the properties of the subsurface.
Filtering methods require determination, or at least an educated guess, of wave propagation velocities in the subsurface media that the reflected seismic waves pass through in their journey from the seismic source to a receiver. Seismic velocity can differ significantly from one type of medium to another.
Some wave-equation methods require structural information, i.e., information about the subsurface structure the determination of which is the reason for doing seismic exploration in the first place. Other wave-equation methods require the shape of the source wavelet which will not be a pure delta function because of reverberations and frequency bandwidth limitation. Some wave-equation methods require both structural and source wavelet information. If the shape of the source wavelet is not accounted for, i.e., is ignored, predicted multiples will not match actual multiples and attempts to cancel out multiples will fail.
From the foregoing, it can be seen that there is a need for a method for accurate prediction of multiples that is fast enough to be applied to large quantities of data such as 3-D marine surveys. Such a method should be simple, robust, efficient, and capable of computer automation. It should not need any subsurface velocity or structure information, or any source wavelet information. Preferably, the method should at the user's choice either provide velocities of the multiples or complete the process of multiple suppression. The present invention satisfies this need.